Studies are continuing toward defining the mechanisms by which oral actinomycetes adhere to other plaque bacteria and to mammalian cells. One important mechanism involves a lactose-inhibitable, cell-associated lectin on human strains of Actinomyces viscosus and A. naeslundii. The results of three independent experimental approaches have associated the lectin activity on these bacteria with fimbriae (i.e. pili) of a single antigenic type, designated Ag 2. These results involve: (i) detection of the lectin activity of purified Ag 2 fimbriae; (ii) blocking of lactose-sensitive adherence by Fab fragments against the Ag 2 fibriae; and (iii) the complete absence of Ag 2 fimbriae on mutant bacterial strains which are specifically defective in lactose-sensitive adherence. In addition to Ag 2 fimbriae, many actinomycete strains have been shown to possess fimbriae of an additional antigenic type, designated Ag 1. Several monoclonal antibodies have been produced against the Ag 1 structures and have served as reagents for purifying these components and for studying their functions. The Ag 1 fimbriae lack detectable lectin activity but may contribute to certain adherence phenomena which are not inhibited by lactose. Thus, distinct structures on the bacterial cell-surface seem to perform specific adherence functions.